calculate the speed of a ball, with a laser

Question

I'm doing a project where I have to calculate the speed of a ball, passing a certain point.

I want to do this with a laser, by taking the time the laser is broken in comparison of the ball it's size.

Someone who can help me with the correct formula?

Answer 1

The laser will "draw" a line segment of apparent length $l$ on the bullet, during time $t$.

The exact formula is

$$v_\perp=\frac lt,$$ where $v_\perp$ denotes the component of the average speed perpendicular to the beam.